An internal combustion engine with multiple cylinders is subject to imbalances in air/fuel ratio between the cylinders due to unequal distribution of air, fuel, exhaust gas recirculation (‘EGR’) gas, and evaporative purge vapor into each cylinder. This unequal distribution is influenced by system limitations, e.g. design constraints of intake and exhaust manifolds, and by component limitations, e.g. variations in part-to-part performance and variations in performance throughout life of each part. The imbalance may vary across engine operating conditions of engine speed and load. Engine designers and calibrators use various engine control strategies to accommodate cylinder-to-cylinder air/fuel ratio imbalances to in order to develop engines and engine control systems that achieve requirements for tailpipe emissions, driveability, performance, and fuel economy. Modern engine control strategies include various hardware systems and algorithms to manage fuel control, spark timing, intake and exhaust valve lift and timing, introduction of EGR gas, and, introduction of vapor purge, to achieve the aforementioned requirements.
There is a continuing demand for improved fuel efficiency of internal combustion engines. One set of strategies being implemented to improve fuel efficiency comprises varying opening time and lift of engine intake valves to manage engine breathing under varying operating conditions including engine cold start, engine idle, partial throttle conditions, steady state operation, and high speed/load conditions.
One strategy that has been successfully implemented to identify and adjust engine control to accommodate cylinder-to-cylinder variations is referred to as individual cylinder fuel control (‘ICFC’). One implementation of an ICFC strategy comprises an algorithm implemented as part of an overall engine control strategy to monitor air/fuel ratio performance of each individual cylinder using input from an exhaust gas sensor. The exemplary ICFC algorithm determines a correction factor for each cylinder that accommodates imbalance in the specific cylinder. The correction is in the form of a modification of actuation signal (e.g. base pulsewidth) to a fuel injector corresponding to each cylinder. An example of an ICFC strategy has been previously described in U.S. Pat. No. 6,382,198 B1: Individual Cylinder Air/Fuel Ratio Control Based on a Single Exhaust Gas Sensor, issued to Smith, et al.
Engine designers have implemented various technologies to accomplish improved engine breathing, including variable valve lift control (‘VLC’), variable cam phasing (‘VCP’), and cylinder deactivation systems. One particular technology includes reducing intake valve lift under light load conditions, to reduce engine pumping losses. Technologies implemented to reduce intake valve lift include, for example, two-step valve lifters, wherein each valve lift actuator is operable open each valve to a low-lift position or a high-lift position. Two-step valve lifter systems typically have a common actuation scheme, wherein a controller activates a common oil control valve. However, each engine valve typically has an individual two-step lifter mechanism that may malfunction. A malfunction of a single individual two-step valve lifter leads to poor driveability, deterioration of engine performance, degradation of emissions performance, engine misfire, damage to engine components, and other events that are deleterious. However, a malfunction of a single individual two-step valve lifter is difficult to detect and diagnose, due to a lack of an effective monitoring system.
Therefore, there is a need to be able to accurately diagnose when one or more variable lifters fails to operate as intended, to reduce risks of engine performance degradation and engine damage. There is a further need to accomplish this task, without addition of sensing devices with accompanying wiring harness, connectors and other hardware, to the engine.